Chapter 3. Results and Discussion
1. Explored optimal condition of fermentative FWL
1.1 Effect of pH and HRT on VFAs production
From 9 different operation conditions designed by RSA, total 11 times of continued fermentation process were operated by utilizing the FWL as substrate. Concentration change of fermentation product for the operation period in accordance with different pH for HRT 4 days condition is shown in Figure 7.
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Figure 7. VFAs production profile at 4 day HRT (A): pH 4.5, (B): pH 5.5, (C): pH 6.5
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From the condition of pH 4.5, acetic acid and ethanol could be observed as prevailed, and very low VFAs production was observed when compared to different pH conditions.
When pH was 5.5, the level of butyric acid was reached to its maximum during the middle phase of operation period, however, it was rapidly reduced after the 3 turn over, and main portion of total volatile fatty acid were caporic acid and acetic acid.
Finally, from the condition of pH 6.5, the highest level of total volatile fatty acid was produced, and large quantity of long-chain fatty acid was generated when compared to other conditions. As the composition of fatty acid, butyric acid and caporic acid could be confirmed as prevailed. In the case of the ethanol as a fermentation product, high con centration was steadily maintained regardless of pH.
The second test was progressed at HRT 2.5 days, and in this case, total 3 times of tests were conducted at pH 5.5 condition which was the central point in accordance with central composite design method and they are shown in Figure 8.
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Figure 8. VFAs production profile at 2.5 day HRT (A): pH: 4.5, (B) – (D): pH: 5.5, (E): 6.5
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From this operation, as in the previous case of HRT 4.5 days, when pH was 4.5, the lowest TVFAs production yield was indicated, and so was the same pattern for the composition.
From the condition of pH 5.5, slightly different aspect from the previous test could be observed, and for the 3 times of test, the highest portion was occupied by butyric acid, and the generation quantity of the fatty acids (valeric acid, caporic acid) with longer chain structure was conspicuously increased. This is considered to be caused by the problem of maintaining the pH level which was generated during the progress of the previous test.
Finally, the highest portion was occupied by butyric acid even from the condition of pH 6.5, and followed by the mass generation of propionate. The highest quantity was indicated even at this moment.
The result of HRT 1 day condition is indicated in Figure 9 with graph and even in this case, low yield, acetic acid, and ethanol were prevailed at pH 4.5. At higher pH, the quantity of butyric acid was the highest, and from the condition of pH 6.5, as in the case of the previous result, propionate was the next.
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Figure 9. VFAs production profile at 1 day HRT (A): pH 4.5, (B): pH 5.5, (C): pH 6.5
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Therefore, it could be confirmed that the composition and pattern of fermentation product in accordance with each of the reactor operation conditions were somewhat divided in accordance with pH, and it could be estimated that the production yield of fermentation product became larger at higher pH. In addition, it could be confirmed that the net-production was not indicated by ethanol when comparing the quantity contained in the substrate used for this test (20.8 g/L) and the quantity of 13.6
± 1.5 g/L which was the result of averaging the result values of all tests regardless of fermentation condition.
On the basis of the data under steady state for each of the operation conditions, the outline of total volatile fatty acid (C2-C6) excluding ethanol is shown in Figure 10. Based on this figure, the lowest production quantity of total volatile fatty acid could be identified (8.6-12.0 COD g/L) from the test with pH 4.5, and at this moment, it could be confirmed that the portion of acetic acid was more than 93%. In addition, it could be understood that this acetic acid had the tendency of decreasing (10-47%) as pH was increased, and the portion of long-chain VFAs was increased and the increasing quantity of butyric acid was conspicuous.
Figure 10. VFA composition at steady state in the FWL fermentation runs tested under different HRT and pH condition
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This result is corresponded with the result of the previous research on acid fermentation that acetic acid and butyric acid are prevailed over the product of volatile fatty acid. (Ren et al. 2007)It can be explained that the change of metabolic pathway is reflected to the difference of the characteristics of fermentation, and at least in this area, it is appeared that influence is exerted to organic acid production as influence is directly applied to the activation of key enzyme by Ph (Feng et al. 2009) In addition to this, the pKa value of organic acid is known to be around 4.8, and the suppression of the production of organic acid at pH 4.5, and the toxicity of unassociated VFAs in connection with HRT may have exerted adverse influence to the growth of microorganism by permeating the cell membrane.(Warnecke and Gill 2005)
Although the decomposition of far more substrate is induced by the long HRT in theory, it is not necessarily required for the increase of organic acid production.
From the test conducted under the condition of pH 6.5, the highest level of VFA quantity (33.9 - 36.7 COD g/L) could be observed, and this was by far the higher value when compared to other conditions. This result is corresponded with the result of previous study that maximum value is indicated at pH 6 which is near to the neutrality for generating organic acid by utilizing food waste. This result can be explained in connection with the pH dependence portion of hydrolase as the activation point of this enzyme is near to pH 6. (Jiang et al. 2013)
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